Deflection surveying — to directly observe a right deflection angle at a station, how should the theodolite be set on the back station reading before turning to the forward line? Choose the most appropriate setting.

Introduction / Context:
In route surveying and curve setting, deflection angles are frequently used to turn the forward direction from a back tangent. The reading you set on the back station determines whether the deflection to the new line is read directly and with the correct sense (right/left). This question reinforces the standard field convention for measuring a right deflection cleanly and unambiguously.

Given Data / Assumptions:

• The instrument is centered and leveled at the point of deflection (station).
• The back station (back tangent) is sighted first to establish reference.
• A right deflection to the forward line must be read directly from the circle.

Concept / Approach:
When the vernier/circle is set to 180° while sighting the back station, a clockwise turn to the forward line yields the right deflection angle as a direct reading. This convention minimizes sign confusions and aligns with many field books that list right deflections as positive clockwise rotations from the back tangent.

Step-by-Step Solution:

Verification / Alternative check:
Field practice notes show that setting 0° on the back station typically makes left deflections read directly; setting 180° makes right deflections read directly. Crews maintain this habit to avoid transcription errors.

Why Other Options Are Wrong:

• Zero on back station: Suits left deflections; not the standard for direct right deflection readings.
• 90°/270° on back station: Arbitrary and do not map the forward direction to the deflection directly.

Common Pitfalls:
Failing to re-zero or set 180° consistently between stations, causing sign flips in the field book.

Final Answer:
180° on the back station